# Modeling Momentum: Day 3

Today’s quotes:

• “That’s really good data!”
• “I think that’s what we had wrong…our equation.”
• “I think you have too many dog pictures on your phone. That’s why you can’t find the data.” (student had taken a picture of their whiteboard yesterday but couldn’t find it today)
• “Hey guys, what if we add in acceleration?” (They didn’t. I didn’t say anything, the other group members shot him down)
• “Yes. Yes!”

Today I started class by showing them that they can measure initial and final velocities but asking “How else can we represent them?” Crickets. “Change in velocity?” Change in velocity! That’s your hint for the day. Go.

One of the things I have found interesting in this process is the many ways students find to say exactly the same thing. The college of ‘correct’ models below demonstrates this.

By the end of the hour, with a 3 word hint, every group had developed something that represents conservation of momentum. Once a group had a model that was either valid or close to it I gave them another situation to test. What if both cars are moving to start with? What if they go in the same direction? What if you use one magnet and one Velcro so that they don’t bounce perfectly yet don’t stick together?

Tomorrow I am going to have them start by having each group put their models on the board to compare. I think it will be powerful for them to see that they all have essentially the same thing with different representations.

Then I plan to scare them a bit. What if both carts are at rest to begin with? Explosions!

I expect that they will think they need to revise their models again. I’m hoping they will be pleasantly surprised when their current models still work.

For the second half of class I am hoping I can guide them from Newton’s 2nd to conservation of momentum. I have been talking a lot about the two main ways to do physics; 1) Take a bunch of data and then find a relationship (Kepler using Tycho Brahe’s data is a great example of this, and the kids love hearing about Tycho’s nose, dwarf, and elk), 2) Start with known physics and try to deductively find a new relationship. We just got done with method 1, let’s see if we can get the same thing with method 2. I plan to use the following instructions to guide them to this point;

• Draw force diagrams for both carts when they collide.
• Write Newton’s second law for both carts.
• Play with the equations.
• Go.

I have high hopes, as always.